Field of the Invention
The present invention relates to an improved process for the monoalkylation of ethylbenzene with isobutene or the monoalkylation of isobutylbenzene with ethylene to form tertiarybutylethylbenzene of high purity, and which process is highly selective towards production of the para isomer.
Description of Related Art Para-tertiarybutylstyrene (Para TBS) is a well known monomeric material useful in the preparation of polymers and copolymers which are in turn useful as ingredients in paint and coating compositions, as well as in the production of thermoset resins.
Para-TBS is produced by the dehydrogenation of the para-tertiarybutylethylbenzene (para TBEB) precursor material using conventional dehydrogenation techniques. Para TBEB is itself most commonly produced by the alkylation of ethylbenzene with isobutene in the presence of an alkylation catalyst.
Mixed meta and para TBEB may be typically prepared by the catalytic alkylation of ethyl benzene with isobutylene in the presence of a Friedel-Crafts type catalyst such as sulfuric acid, BF.sub.3, aluminum trichloride, liquid hydrofluoric acid as well as mixtures thereof, such as disclosed in U.S. Pat. Nos. 3,631,213 and 4,982,034.
Whereas these processes produce TBEB in relatively good yields, they generally give rise to a mixture of the para and meta isomers of TBEB which are difficult to separate out because of their relatively close boiling points. The most valuable and useful isomer is the para isomer since its dehydrogenation product (para TBS) is the most desired monomer for use in the polymer applications described above.
Crystalline aluminosilicate catalysts, or zeolite catalysts, are also known to be useful to catalyze the alkylation of aromatic compounds using olefins as the alkylation agent. For example, U.S. Pat. No. 4,469,908 discloses the alkylation of aromatic hydrocarbons by contacting a mixture of aromatic hydrocarbon and a lower olefin, present in a mole ratio in the range of 20/1 to 1/1 respectively, with a crystalline zeolite catalyst at a temperature of between 100.degree. C. and 300.degree. C. and under sufficient pressure to maintain the organic reactants in the liquid phase. The alkylation of ethylbenzene with isobutylene using the acid form of ZSM-12 Zeolite is particularly disclosed wherein greater than 95% conversion of isobutylene is achieved with a selectivity of 90% towards para TBEB and 10% towards meta TBEB.
In spite of the availability of highly selective zeolite catalysts which are active for these alkylations, the production of alkylated aromatic compounds frequently suffers from undesirable side reactions which cause the formation of unwanted isomers and impurities. For example, exposure of a reaction stream which includes isobutene and ethylbenzene to conditions of temperature and pressure sufficient to induce catalytic alkylation with high product yield may cause some cracking and isomerization of the reaction product as well as undesired isobutene oligomerization, and the formation of some of the unwanted trialkyl isomers of TBEB.
It is generally known in the art that the quantity of unwanted polyalkyl benzene by-products produced during monoalkylation can be minimized by conducting the alkylation using relatively high molar ratios of aromatic substrate to alkylating agent and/or by separating the unwanted polyalkylaromatics from the monoalkylation product and recirculating the former back to the reactor to undergo transalkylation.
In one particular embodiment of an alkylation process, a mixture of olefin, e.g. ethylene, aromatic substrate, e.g., benzene, and catalyst, e.g., aluminum chloride, are pumped into a tank reactor, mixed and subjected to conditions of temperature and pressure sufficient to cause alkylation of the aromatic substrate with the olefin. A portion of the reaction mixture containing the reaction product, unreacted aromatic substrate, some unreacted olefin and catalyst is continuously withdrawn from the reactor, cooled, mixed with fresh ethylene and benzene, and pumped back into the reactor as a method of controlling the temperature of the exothermic reaction in the reactor and enhancing mixing of the reactants. However, this process suffers the disadvantage of the need to neutralize, remove and dispose of the aluminum chloride catalyst as well as the difficulty in controlling precisely the olefin to aromatic substrate molar ratios throughout the reaction mass.
U.S. Pat. No. 3,766,290 discloses a process for preparing alkylated aromatics comprising contacting a mixture of alkylating agent, aromatic hydrocarbon and polyalkylated aromatic hydrocarbon with an alkylation catalyst in a reactor vessel under alkylation conditions, separating the crude reaction product from the catalyst, separating the alkylated aromatic hydrocarbon from the unconverted aromatic hydrocarbon, further separating the monoalkylated aromatic hydrocarbon from the polyalkylated aromatic hydrocarbon and recycling the aromatic hydrocarbon and polyalkylated aromatic hydrocarbon back to the reactor vessel.
U.S. Pat. No. 4,169,111 discloses a process for the manufacture of ethylbenzene wherein a mixture of ethylene and excess benzene is contacted with an alkylation catalyst in a reactor to produce a product comprising a mixture of ethylbenzene, diethylbenzene and triethylbenzene, separating the product into benzene and each of the alkyl and polyalkylbenzene fractions, and recycling at least a portion of the separated diethylbenzene fraction back to the reactor.
The prior art also discloses several techniques for conducting alkylation reactions where high molar ratios of aromatic substrate to alkylating agent are achieved. For example, U.S. Pat. No. 4,849,569 teaches that enhanced selectively towards the production of monoalkylbenzenes can be achieved by providing a mole ratio of benzene to olefin alkylating agent in the ratio of 2 to 100 : 1, more preferably from about 2 to 10 : 1. However, a major disadvantage of this process involves the separation and recovery of large volumes of unreacted aromatic hydrocarbon which lends inefficiency and increased cost to the process.
Another technique to obtain the advantages of the use of relatively high aromatic substrate/alkylating agent mole ratios is to conduct the alkylation process in a multibed, multistage reactor wherein only a portion of the total moles of alkylating agent is injected at each stage of the reactor as the reaction stream passes through the reactor. Examples of such techniques are disclosed in U.S. Pat. Nos. 4,922,053 and 5,073,653. However, the use of such systems requires more elaborate process temperature control and catalyst handling procedures in order to avoid catalyst deactivation caused by rapid build up of coke on the catalyst surface.